8914
J . Am. Chem. SOC.1989, 1 1 1 , 8914-8916
Our optimized conditions for promoting the oxidative cyclization of substrates 6 and 12 (Scheme 111) are derived from a related transformation initially reported by Yamamura17 and later modified by I n ~ u e . ' ~ *We ' ~ have found that the optimal protocol for the cyclization of 6 is oxidation with 10 equiv of thallium(II1) nitrate trihydrate (TTN) (excess TTN is necessary to ensure complete reaction) in 5:l THF/methanol at 1 mM concentration with 3 equiv of pyridine/equiv of TTN to serve as an acid scavenger. Increasing the ratio of T H F to methanol results in incomplete reaction, while increasing the ratio of methanol to T H F results in a lower yield. The reduction of the resulting para-quinol 13 is accomplished in situ by the addition of excess CrC12.20 We have found these conditions to be superior to the zinc/acetic acid reduction described by Yamamurat9as they avoid the isolation of the unstable intermediate para-quinol methyl ether. Under the conditions described above, the cyclic product 14 is isolated in 42% overall yield from the cyclization precursor 6. Model peptide 12 was cyclized and subsequently reduced under analogous conditions except that 1 :1 CH2C12/methanol was employed as the solvent. When these conditions were employed, the macrocyclic diphenyl ether 16 was obtained in 48% overall yield. One notable difference between the two macrocyclizations is the displacement of bromine by methoxide in the formation of para-quinol 15.2' Presumably, this substitution occurs in the cyclization of 12 and not in the cyclization of 6 due to a more sterically crowded environment at the para position of the intermediate leading to 15. These observations thus dictate the order of assemblage of the macrobicyclic diether 19. In order to evaluate the oxidative coupling strategy to provide the C, D, E bicyclic phenyl ether vancomycin synthon, the monocyclic diphenyl ether 14 was treated with trifluoroacetic acid to remove the Boc protecting group, and the resulting amine was coupled to tripeptide 11 with diisopropylcarbodiimide and hydroxybenzotriazole to provide the hexapeptide 17 in 72-78% overall yield (Scheme II1).l6 The allyl group was then removed as described previously in 92-93% yield to provide hexapeptide 18. The optimal conditions for the cyclization of 18 were found to be 5 equiv of T T N in 30:l methylene chloride/methanol at 1 mM concentration (4 h, -23 "C). After in situ reduction of the resulting para-quinol ether with excess CrCI2, the dicyclic compound 19 was obtained in 40% overall yield. These studies clearly demonstrate the feasibility of pursuing a total synthesis of vancomycin and related antibiotics via biomimetic oxidative phenolic coupling. Acknowledgment. Support has been provided by the National Institutes of Health, Eli Lilly, and Merck. The N I H BRS Shared Instrumentation Grant Program 1 S10 RR01748-01A1 is acknowledged for providing N M R facilities. J.A.E. and K.M.D. acknowledge support as N S F Predoctoral Fellows. Supplementary Material Available: Spectral data for all compounds and detailed experimental procedures for the oxidative macrocyclizations as well as for the syntheses of 2-6, 17, and 18 (15 pages). Ordering information is given on any current masthead page. (16) A diastereomeric compound, 7%, was also produced in this coupling reaction. (17) Nishiyama, S.; Nakamura, K.; Suzuki, Y.; Yamamura, S. Tetrahedron Lett. 1986, 27, 4481-4484. (18) Inaba, T.; Umezawa, I.; Yuasa, M.; Inoue, T.; Mihashi, S.; Itokawa, H.; Ogura, K. J . Org. Chem. 1987, 52, 2957-2958. ( I 9) More recently, Yamamura has employed these conditions in the synthesis of OF4949-I11 (Nishiyama, S.; Suzuki, Y.; Yamamura, S. Tetrahedron Lett. 1988, 29, 559-562) and in the synthesis of K-13 (Nishiyama, S.; Suzuki, Y.; Yamamura, S. Tetrahedron Lett. 1989, 30, 379-382). (20) Hanson, J. R.; Mehta, M. S. J . Chem. SOC.,Chem. Commun. 1969, 2349-2350. (21) In the oxidation of an analogue of 6 containing the Cbz protecting group rather than the Boc protecting group, approximately 5-10% of the methoxy-substituted derivative of 14 was also observed. In the limited number of systems that we have studied, increasing the ratio of T H F to methanol and addition of pyridine increases the ratio of the bromo- to the methoxy-substituted cyclic products.
0002-7863/89/ 15 1 1-89 14$01.50/0
Aristolochene Biosynthesis and Enzymatic Cyclization of Farnesyl Pyrophosphate David E. Cane,* P. C. Prabhakaran, Edward J. Salaski, Paul H. M. Harrison, Hiroshi Noguchi, and Bernard J. Rawlings Department of Chemistry, Brown University Providence, Rhode Island 0291 2 Received July 21, 1989 Aristolochene (1) is a bicyclic sesquiterpene belonging to the eremophilane group of hydrocarbons. The (-) enantiomer of 1 was first isolated in 1970 by Govindachari et al. from the plant Aristolochia indica.' It is also reported to occur in Bixa orellana leaf oil and in the defensive secretions of Syntermes soldier term i t e ~ . The ~ ~ ~(+) enantiomer 1 was recently isolated in our laboratory from the mycelial extracts of the fungus Aspergillus t e r r e ~ s .The ~ (+) enantiomer is also the probable biosynthetic precursor of PR toxin produced by Penicillium roquefortii.$ Recently, Hohn and co-workers have isolated aristolochene synthase from P. roquefortii6 and purified the enzyme to homogeneityS7 The proposed mechanism for the formation of aristolochene from farnesyl pyrophosphate (FPP) (2), the universal precursor of sesquiterpenes,I0 is shown in Scheme I. Cell-free extracts of A . terreus prepared from mycelia harvested between 45 and 60 h after inoculation showed terpenoid cyclase activity." Preparative incubation of [1-3H]FPP (2a)12with crude cell-free extracts produced radioactive hydrocarbon la,13 which was found to cochromatograph with synthetic (f)-arist~lochene'~ on TLC (SO2, AgN03-Si02) as well as by radio-GC analysis (FFAP). Dilution with carrier (f)-aristolochene and oxidation with MCPBA followed by hydrolysis with HCIO4 gave the corresponding diol 3a, which was recrystallized to constant activity, thereby confirming (1) Govindachari, T. R.; Mohamed, P. A,; Parthasarathy, P. C. Tetrahedron 1970, 26, 615. (2) Lawrence, B. M.; Hogg, J. W. Phytochemistry 1973, 12, 2995. (3) Baker, R.; Cole, H . R.; Edwards, M.; Evans, D. A.; Howse, P. E.; Walmsley, S. J . Chem. Ecol. 1981, 7, 135. (4) (a) Cane, D. E.; Rawlings, B. J.; Yang, C.-C. J . Antibior. 1987, 40, 1331. (b) Cane, D. E.; Salaski, E. J.; Prabhakaran, P. C. J . Antibiot., submitted for publication. (5) (a) Moreau, S.;Lablache-Combier, A,; Biguet, J. Phytochemistry 1981, 20, 2339. (b) Gorst-Allman, C. P.; Steyn, P. S. Tetrahedron Letr. 1982, 23, 5359. (c) Chalmers, A. A,; DeJesus, A. E.; Gorst-Allman, C. P.; Steyn, P. S. J . Chem. SOC.,Perkin Trans. I 1981, 2899. ( 6 ) Hohn, T. M.; Plattner, R. D. Arch. Biochem. Biophys. 1989,272, 137. ( 7 ) The epimeric sesquiterpene, (+)-5-epiaristolochene, has been demonstrated to be an intermediatesain the biosynthesis of the phytoalexins capsidiol and debneyoLs8 It has recently been shown that treatment of tissue cultures of Nicotiana tabacum with various elicitors results in appearance of a cyclase capable of converting FPP to a sesquiterpene hydrocarbon identified as (+)-5-epiaristoIo~hene.~ (8) (a) Threlfall, D. R.; Whitehead, I. M. Phytochemistry 1988, 27, 2567. (b) Brooks, C. J. W.; Watson, D. G.; Rycroft, D. S.; Freer, I. M. Phyrochemistry 1987, 26, 2243. (c) Hoyano, Y.; Stoessl, A.; Stothers, J. B. Can. J . Chem. 1980,58, 1894. (d) Baker, F. C.; Brooks, C. J. W. Phytochemistry 1976, IS, 689. (e) Baker, F. C.; Brooks, C. J. W.; Hutchinson, S. A. J . Chem. Soc., Chem. Commun. 1975, 293. (9) (a) Whitehead, I. M.; Threlfall, D. R.; Ewing, D. F. Phytochemistry 1989,28,775. (b) Vogeli, U.; Chappel, J. Plant Physiol. 1988,88, 1291. (c) Whitehead, I. M.; Prabhakaran, P. C.; Ewing, D. F.; Cane, D. E.; Threllfall, D. R. Phytochemistry, in press. (10) (a) Cane, D. E. Acc. Chem. Res. 1985, 18,220. (b) Cane, D. E. In Biosynthesis of Isoprenoid Compounds; Porter, J. W., Spurgeon, S. L., Eds.; John Wiley: New York, 1981; Vol. 1, p 284. (11) Croteau, R.; Cane, D. E. Methods Enzymol. 1985, 110, 383. (12) Preuared as previously described: Cane, D. E.; Iyengar, . - R.; Shiao, M.-% J . Am. Chem.'Soc. 1981, 103, 914. (13) The mycelia from 500 mL of 45-h culture were used to prepare the cell-free extract" in 190 mL of buffer (10 mM Tris, 5 mM MgCII-6H20, 5 mM /3-mercaptoethanol, and 15% v/v glycerol adjusted to pH 7.8 with 6 N HCI). [1-3H]FPP (8.8 X lo' dpm, 7 mmol) was incubated with 2 mL of crude extract at 30 OC for 2 h, and the resulting radioactive hydrocarbon (4.0 X lo4 dpm) was extracted into pentane, passed through a small SiO, column, and concentrated by Vigreux distillation. (14) Piers, E.; Geraghty, M. B. Can. J . Chem. 1973, 51, 2166.
0 1989 American Chemical Society
J . Am. Chem. SOC.,Vol. 111, No. 24, 1989 8915
Communications to the Editor Scheme I
B
1 / k . 12
13
r+y+ PPO
I,
Scheme I1 OH
6 0
4 0 PPH
2 0
Figure 1. Deuterium N M R spectra (61.4 MHz) of aristolochene l e (300 nmol) and If (480 nmol) derived from (A) (1R)-[l-2H]FPP(Ze) and (B) (lS)-[1-2H]FPP (20. Shifts are relative to natural abundance CDCI, at 6 7.24.
Scheme 111 1. BuLi
1
IC
Id
the structure of the enzymatic cyclization product. When an analogous incubation was carried out using [ 12,13-14C]FPP(2b),I2 selective oxidation (Os04 followed by N a I 0 4 ) of the derived aristolochene (lb) gave the methyl ketone 4 (semicarbazone, 6.61 X IO4 dpm/mmol), which retained one-half the I4C label present in the corresponding diol 3b (1.28 X lo5 dpm/mmol), as expected (Scheme 11). Unambiguous evidence for the distribution of isotopic label in the cyclization product was obtained by incubation of 5.75 Kmol of a mixture of [ l l,12-l3C2]-and [ l 1,13-13C2]FPP(2c,d), prepared as shown in Scheme 111and containing [12,13-14C]FPPas internal standard, with crude aristolochene synthase at 30 OC for 3 h, yielding 370 nmol of aristolochene. After addition of carrier aristolochene (8 mg) to the crude pentane extract, the labeled aristolochene (lc,d) was purified by Si02column chromatography (pentane) followed by AgN0,-SO2 preparative TLC (solvent: 70% hexane, 30% benzene) and then analyzed by 100.61-MHz I3C NMR. The peak corresponding to C-11 (6 150.64 ppm) appeared as a pair of doublets flanking the natural abundance singlet ( J (I3C-I3C) = 72 and 42 Hz), while the resonances corresponding to C-12 (6 108.27 ppm, J = 72 Hz) and C-13 (6 20.84 ppm, J = 42 Hz) each appeared as enhanced doublets.
In order to establish the stereochemistry of the initial cyclization at C-1 of FPP, we required unambiguous assignments of the IH N M R chemical shifts of the geminal protons at C-6 of 1. On the basis of 'H-IH COSY and lH-13C HETCORR spectroscopy, the signal at 6 1.76 ppm (dt, J = 13.6, 4.0 Hz, geminal coupling to H-6,,, vicinal coupling to H-7,, and "W" coupling to H-8,) was assigned to the 6-equatorial proton and the signal at 6 1.17 (t, J = 13 Hz, geminal coupling to H-6,, vicinal coupling to H-7,,) was assigned to H-6,,. These assignments were supported by the results of difference NOE spectroscopy. Thus, irradiation at 1.76 ppm led to enhancement of the proton resonances at 2.25 (H-7, 3.8%), 1.17 (H-6,, 15%), 0.96 (H-15, 2.5%), and 0.83 ppm (H-14, 3.5%) whereas irradiation at 0.83 ppm resulted in enhancement of the peaks at 1.76 (H-6 , 2%) and 0.96 ppm (H-15, 1.5%). Furthermore, irradiation o? the signal at 0.96 ppm gave rise to enhancements at 2.25 (H-7, 3.0%), 1.76 (H-6,, I.2%), and 0.83 ppm (H-14, 1.7%). The observed NOES were consistent with the conformation of aristolochene calculated by using the MacroModel molecular modeling program and an MM2 force field. Both (1R)- and (lS)-[l-zH]FPP (2eand 2f)ISawereseparately incubated with crude aristolochene synthase from A. terreus, and the purified product from each incubation was analyzed by 61.42-MHz 2H NMR spectroscopy. Aristolochene (le) obtained from (1R)-[1-2H]FPP exhibited a single peak at 6 1.76 ppm corresponding to H-6, (H-6,e) (Figure 1) and that (If) from (lS)-[1-2H]FPPshowed a single peak at 6 1.17 ppm corresponding to H-6,, (H-6,J of aristolochene. These results clearly indicate that the cyclization of FPP to aristolochene is proceeding with inversion of configuration at C-1 of FPP, consistent with the mechanism illustrated in Scheme I. Similar results have been obtained for the enzymatic formation of pentalenene, involving initial cyclization of FPP through an 11-membered-ring, rather than a IO-membered-ring,intermediate.Isa By contrast, cyclization of FPP to 6-membered-ring products requires initial isomerization to the tertiary allylic isomer nerolidyl pyrophosphate and has been shown to result in net retention of configuration at C-1 of the allylic pyrophosphate substrate.lsbJ6J7 (15) (a) Harrison, P. H. M.; Oliver, J. S.;Cane, D. E. J . Am. Chem. SOC. 1988, 110, 5922. (b) Cane, D. E.; McIlwaine, D. B.; Harrison, P. H. M. J . Am. Chem. SOC.1989, 1 1 1 , 1152. (16) Cane, D. E.; Ha, H. J.; Pargellis, C.; Waldmeier, F.; Swanson, S.; Murthy, P. P. N. Bioorg. Chem. 1985, 13,246. Cane, D. E.; Ha, H. J. J . Am. Chem. SOC.1988, 110, 6865. (17) See also: Croteau, R. Chem. Reu. 1987, 87, 929.
J . Am. Chem. SOC.1989, 1 1 1 , 8916-8917
8916
Acknowledgment. This work was supported by grants from the National Institutes of Health, GM22172 and GM30301. We also thank Dr. Thomas Hohn of the USDA, Agricultural Research Laboratory, in Peoria, IL, for sharing with us his results on the isolation of aristolochene synthase from P. roquefortii in advance of publication.
l
a
300
400
b
Square-Planar Complexes of Platinum(I1) That Luminesce in Fluid Solution Juan A. Zuleta, Carlos A. Chesta, and Richard Eisenberg* Department of Chemistry, University of Rochester Rochester, New York 14627 Received May 19, 1989 Very few mononuclear complexes of square-planar geometry luminesce in fluid solution,1,2principally because efficient radiationless decay occurs via collisions with solvent in the open coordination sites. Of the few d8 complexes that do emit in fluid solution, only the cyclometalated species P t ( t h ~ y (thpy ) ~ = 2(2-pyridy1)thiophenide) possesses an emitting state showing metal in~olvement.~Other luminescent Pt(I1) complexes either are not only as solids at low t e m p e r a t ~ r e .In~ ~ m ~ n o n u c l e a or r ~ emit ~ this paper we describe two new R(I1)complexes that exhibit strong solution luminescence, show solvatochromic behavior, and undergo electron-transfer quenching with both donors and acceptors. These complexes are members of a larger class of solution luminescent dithiolate diimine ds systems5 The complexes Pt(Nrhl)(Eda), where ecda = ethyl 2-cyano3,3-dimercaptoacrylate and N N = 4,4’-dimethyl-2,2’-bipyridine ( l ) ,and 4,7-diphenyl-l ,IO-phenanthroline (2) were prepared via eq 1 from Pt(
